This invention relates to an electro-optic rearview mirror system for a vehicle.
Electro-optic rearview mirrors are very well known in the art and include a mirror housing containing a variable reflectance electro-optic mirror unit. While many different constructions of variable reflectance electro-optic mirror units are known, a typical construction uses two substantially parallel glass plates, one of which (herein referred to as the rear plate) is coated on one surface with a reflective layer (the other plate is referred to as the front plate). The front and rear plates are separated by a space which contains an electro-optic medium allowing variation in the amount of light transmitted through the medium by varying the strength of an electric field applied across the medium. For example, in liquid crystal rearview mirrors the space between the front and rear plates is filled with a semi-viscous liquid crystal material. In electrochemichromic or electrochromic mirrors, the space contains a liquid, thickened liquid, gel or semi-solid material.
The mirror unit is mounted in the mirror housing with the front plate facing the viewer, so that light incident externally or the mirror unit returns to the viewer, after reflection at the rear plate, only after a double crossing of the electro-optic medium. Electrode means, for example a respective transparent electrode layer on each plate, permit varying the electrical field across the electro-optic medium, thereby to correspondingly vary the level of reflected light returned to the viewer and accordingly the reflectivity of the mirror unit as a whole.
In the simplest case the reflectivity of the mirror unit is switchable manually between a maximum appropriate for daylight use to a minimum appropriate for nighttime use where it is required to reduce the glare from headlights of following vehicles. However, it is also known to mount one or more light sensors in the mirror housing which sense the light levels forwardly and/or rearwardly of the vehicle and automatically adjust the reflectivity of the mirror unit according to selected criteria. Typical arrangements of the latter kind are described in U.S. Pat. Nos. 4,793,690, 4,799,768, 4,886,960, 5,193,029, 5,140,455 and 5,151,816.
Hitherto, in all cases the control circuitry for the electro-optic mirror unit has been contained, usually on a printed circuit board (PCB), within the mirror housing itself.
However, this has substantial disadvantages. The mirror housing needs to be fairly bulky to accommodate the control circuitry, which is undesirable from an aesthetic point of view, and this is exacerbated by the current trend to located more and more function in the mirror housing such as a compass display, navigational display system and infra-red (IR) sensor for automatic locking system. Also, the control circuitry tends to generate substantial heat, often as high as 2-3 watts, which means the mirror housing has to be designed for adequate ventilation. Further, the mirror is complex to assemble and has a high replacement cost.